1. Field of the Invention
The present invention relates to hybrid mode waveguide and feedhorn antenna and a mode conversion portion of a waveguide and, more particularly, to hybrid mode waveguide and feedhorn antenna and mode converting waveguide comprising a circular waveguide body which for the feedhorn antenna can include a conical section which flares outwards towards the mouth of the feedhorn antenna, and a spiro-helical projection bonded to a dielectric coating on the inner surface of the waveguide or feedhorn antenna. One arrangement for the spiro-helical projection comprises a helically wound dielectrically coated wire which is initially flattened and formed in closely spaced edge-wound turns and gradually changes to a rounded configuration before continuing with linearly increased spacing between the turns to effect mode conversion from the TE.sub.11 mode to the HE.sub.11 mode which changes to a uniform pitch as the helix progresses towards the mouth of the waveguide or feedhorn antenna. A second arrangement comprises multiple layers of dielectrically coated wire in closely spaced turns which gradually reduce to one layer before the spacing between turns is gradually increased in a linear manner to effect the mode conversion and then to a uniform pitch as the projection progresses to the mouth of the waveguide or feedhorn antenna.
2. Description of the Prior Art
Hybrid mode corrugated horn antennas have been in use in the microwave field for a number of years. Various techniques for forming the corrugated horn antennas have been used to provide certain advantages. For example, U.S. Pat. No. 3,732,571 issued to N. W. T. Neale on May 8, 1973 discloses a microwave horn aerial which is corrugated on its inner surface, defining a tapered waveguide mouth area, with at least one spiro-helical projection which can be produced by a screw cutting operation with a single start spiro-helical groove or by molding on a mandrel which can be withdrawn by unscrewing it.
In U.S. Pat. No. 3,754,273 issued to Y. Takeichi et al on Aug. 21, 1973, a circular waveguide feedhorn is disclosed which includes corrugated slots on the inner wall surface, the width of the slots abruptly changing from a smaller value in the portion near the axis of the waveguide to a larger value in the remaining portion of the slot.
In U.S. Pat. No. 4,106,026 issued to N. Bui-Hai et al on Aug. 8, 1978, a corrugated horn of the exponential type is disclosed with corrugations whose depth decreases exponentially from the throat of the horn towards its mouth.
In the typical prior art arrangements, construction is generally complicated and expensive with the possible exception of the Neale feedhorn described hereinbefore, and coupling to a dominate mode waveguide is difficult and limited in bandwidth.
The problem remaining in the prior art is to provide a hybrid-mode feedhorn antenna or waveguide of a design which is inexpensive to fabricate, provides simplified mode coupling of the TE.sub.11 mode to the HE.sub.11 mode, and is operative over a very wide frequency bandwidth.